Rotary friction welding is a process for welding together two bodies or workpieces by converting mechanical energy to heat energy by the friction between the engaging weld surfaces of the two workpieces. The process involves effecting relative rotation between the two workpieces while the weld surfaces remain in engagement with each other.
For example, in inertia friction welding one of two coaxial workpieces is attached to a flywheel, rotated to a predetermined speed and then driven against the second workpiece using thrust supplied by the welding machine. A fixed amount of stored energy in the flywheel (proportional to rpm2·I, where rpm is the flywheel's predetermined speed and I is its rotational inertia) is thereby converted to heat by friction at the interface of the engaging weld surfaces, which bonds the workpieces together.
The initial contact between the weld surfaces produces a conditioning period in which friction raises the temperature at the interface. This is followed by upsetting when the temperature reaches a high enough level such that softening/melting of the workpiece material allows the workpieces to be pushed together, with material being expelled sideways from a plasticised zone at the interface as flash or “upset”.
Problems arise when workpieces of generally circular cross section have non-uniform diameter, for example when together they form a product having an angled or curved circumferential wall. The application of a force in a direction orthogonal to the weld interface in these situations is likely to lead to slippage between the workpieces at the interface with the consequence of weld misalignment. US patent publication 2011/206523 discloses a process for welding co-axially aligned workpieces where the weld interface is angled to both the axis and the orthogonal. Application of both axial and radial force is proposed. It is understood this is necessary to control radial deformation arising from the inclination of the weld interface to the axis. Application of radial force to a pair of workpieces which are configured to form an angled or curved walled product is problematic using conventional rotary friction welding equipment.